Apparatus for suppressing undesirable modes in ultrasonic flowmeters



J. KRITZ APPARATUS FOR SUPPRESSING UNDESIRABLE l April 22, 1958 MODES 1NULTRASONIC FLOWMETERS Y Original Flled Feb. 5,'1951 5 Sheets-Sheet 1April v22, 1958 A J. KRlTz 2,831,348

APPARATUS FOR sUPPREssING UNDESIRABLE MODES IN ULTRASONIC FLOWMETERSoriginal Filed Feb. s, 1951 5 sheets-sheet 2 Ff@ Z.

.A INVENTOR. Jac/r hr/'fz AT TORNEY5 April 22, 1958 J. KRITZ 2,831,348

APPARATUS FOR SUPPRESSING UNDESIRABLE MODES IN ULTRASONIC FLOWMETERSOriginal Filed Feb. 3. 1951 l 5 Sheets-Sheet 4 l V I I I I MIM, waf

ATTOR Aprxl 22, 1958 J. KRlTz 2,831,348

APPARATUS FOR SUPPRESSING UNDESIRABLE MODES IN ULTRASONIC FLOWMETERSOriginal Filed Feb. 3, 1951 5 Sheets-Sheet 5 Fig, 7&7

IN VEN TOR. Jac/'r /frifz A TTORNE K5 United States Patent-'0" APPARATUSFR SUPPRESSIG UNDESIRAL MODES IN ULTRASONIC FLOWMETERS The presentinvention relates to an apparatus for suppressing undesirable modes ofoscillations, and more particularly to an arrangement for suppressingundesirable modes of oscillations occurring in wide band oscillatorysystems, such as ultrasonic flow meters.

Broad band oscillatory systems in which the oscillatory loop includes anelement which has a variable time constant present the problem thatthese systems will oscillate in modes depending on how they happen toget started. Since the frequency of oscillation is not ixed-the elementhaving a variable time constant preventing thisit is not feasible toinclude filter circuits in the oscillatory loop, to determine the modeof oscillation. The use of tilters, which by their nature introduce timedelays, is incompatible with' the broadband (small time delay)requirements.

In the operation of ultrasonic iiow meters, the frequency of oscillationof the oscillatory loop depends on the velocity of ultrasonicpropagation in the fluid, as well as on the flow velocity of the fluidunder consideration. The frequency of such a loop is measured andcompared with the frequency of the loop With the fluid at rest, or withthe frequency of a second, similar loop having a different orientationwith respect to the iiow of the uid than the lirst loop. Flow metersoperating on this principle are described in my copendingapplicationSerial No. 67,503 led December 27, 1948, Serial No. 209,295 liledFebruary 3, 1951 and now abandoned and Serial No. 209,296 tiled February3,1951 and now abandoned, of which application the present case is acontinuation-inpart. in a llow meter having a pair of loops it isimportant that both loops operate on the same known mode: Otherwise,dependable readings cannot be obtained under commercial or eldconditions. t

It is accordingly an object of the present invention to provide anapparatus to suppress undesirable modes in :i

oscillatory loops.

It isa further object of the invention to provide a mode suppressor foruse with a liow meter to suppress undesirable modes which might occurdue to momentary interruptions of the signal received by the receivingtransducer, for example, due to a chattering switch, obstructionspassing by in the stream of the lluid, starting transients, etc.

' With these objects in view, the invention comprises apparatus forsuppressing undesirable modes of oscilla-j tion infan oscillatorycircuitby generating a-n impulse from the oscillatory circuit upon starting ofoscillations;

timing the thus generated impulse to be of longer duration than thetime` required from the start of oscillations to the first premature,undesirable reversal of signal that can be caused by an unwanted mode;and controlling the oscillatory circuitby the thus timed impulse. Thecontrol of the oscillatory circuit may be accomplished by applying thegenerated impulse as a paralyzing signal to the oscillatory circuit, `soas to prevent the starting of undesirable modes of oscillations bymaintaining the conrice dition of the oscillatory circuit in thecondition it was -circuit arrangement to suppress undesirable modes ofoscillations which comprises a detector circuit to detect the signalfurnished by the oscillatory circuit so as to obtain therefrom animpulse upon the start of oscillations. The impulse is then applied to adifferentiating circuit where a signal is obtained in accordance withthe rate of rise of the impulse; and this signal is applied to a timingcircuit to furnish a timed pulse of predetermined duration (synchronizedto start with the rise of the detected signal, i. e. the impulse) whichhowever will be longer under all circumstances than the time requiredfrom the start of oscillations to the first premature, undesirablereversal of signal that can be caused by the undesirable mode. Thistimed pulse is then applied any place in the oscillatory loop (forexample to an amplifier) to prevent a change in the mode of oscillationof the loop, for example by paralyzing the operating of a cornponentthereof for the duration of the timed pulse.

In the accompanying drawings I have shown a mode suppressor according tothe invention, incorporated in, and applied to a flow meter. It is to beunderstood, however, that such a mode-suppressor is equally applicableto other broad band oscillatory loops, and the drawing and detaileddescription refers to the llow meter only by way of illustration of onepossible application.

Figure 1V is a block diagram of a mode suppressor applied to a ow meter;

Figure 2'is a diagram of the wave form in a flow meter under normalcondition;

Figure 3 is a diagram similar to that of Figure 2 showing aninterruption and subsequent sustained generation of an unwanted mode;

Figure 4 is a diagram similar to that of Figure 3, showing the action ofthe mode suppressor in suppressing a disturbance;

Figure 5 is a diagram similar to that of Figure 4 show ing the action ofthe modes suppressor in suppressing a disturbance occurring at a timedilierent from that shown in Figure 4;

Figure 6 is a diagram similar to that of Figure 4 showing the action ofthe modes suppressor in suppressing a third harmonic mode; and Y Figures7 and 7a are a schematic circuit diagram illustrating the detailedcircuit of the mode suppressor applied to a ow meter, Figure 7a being acontinuation of Figure 7 and deemed to be joined thereto at lines x-x.

Referring now specifically to Figure l, the uid whose velocity is to bemeasured is conducted through a tube 11. Transmitting and receivingtransducers 12 and 13 are connected with the tube to transmit supersonicwaves through the liuid medium from transmitter 12 to receiver 13.Carrier excitation is furnished by carrier generator 14 to a modulator15, through which the signal will be conducted to the transmitter 12.This signal will remain on so long as no received signal is impressed onmodulator 15 to turn it off.

When a signal is received by receiver 13, it is amplified by amplilier16, detected in detector 17 and then serves to turn off the signalfurnished to the system by applying a blocking signal to modulator 15.The carrier frequency signal applied to the transmitting transducer 12will then be a square wave of period 2T as shown in Figure 2, where T isthe transit time of ultrasonic vibrations from the transmittingtransducer 12 to the receiving transducer 13. If for any reason afterthe starting time (t=0) the signal is momentarily interrupted, forexample due to such effects as an improperly functioning switch, a loosecontheiiuid stream, then the wave will appear as shown in Figure 3, andwill continue in that form until, another interruption alters itscharacter. As shown, the normal signal A of Figure 2 is now interruptedinto parts A1 and A2 (Fig. 3) by a break B.

The basic oscillatory frequency of Figure 3 is three times that ofFigure 2. For an arrangement as illustrated, any odd mode, 3, 5, 7, etc.times the basic frequency may be started. Such undesirable modes willalways be an 4odd integral multiple of what may be termed thefundamental mode, illustrated in Figure 2.

It is to be noted that the oscillatory circuit of Figure l can also beconstructed so that the modulator normally is off and no excitation issupplied to the transmitting transducer 12 except when a signal isreceived. Such an arrangement will oscillate at a fundamental period ofT (instead of 2T as in the arrangement of Fig. l) and the undesirablemodes willV be even multiples of the fundamental mode, e. g., 2, 4, 6,etc. times higher than the fundamental. It is preferred, however, to usean arrangement according to Figure l, since it is self-starting and theseparation of the nearest mode frequency from the fundamental mode isgreater than for the alternate arrangement just described, i. e., 310-f0=2f0 for the arrangement according to Figure 1, against 210- f0=f0 forthe alternate form, Where fo is the frequency of the fundamental mode.

The mode suppressor is generally indicated at 20, and it is connected bymeans of lead 24 to the oscillatory loop, as shown in Figure l, so thata portion of the signal applied to transmitting transducer 12 is appliedto the mode suppressor 20. The mode suppressor comprises a detector 21,a differentiating network 22, and a pulse generator 23. A sample of thesignal on the transmitting transducer 12 is detected in detector 21, andthen differentiated in dilferentiating network 22 to produce a signalcoincident with the rise of signal voltage on the transmittingtransducer 12. The differentiating network may be, for example, aresistance-capacitance network, or a resistance-inductance network,blocking oscillators, or other circuits which will produce a signalinitiated by a rise of voltage, as is well known in the art. Aresistancecapacitance network is preferred, due to its simplicity. Thissignal is utilized to produce a pulse of known time duration. Thecircuit producing this timed pulse may be a monostable multivibrator,and is so designed that the time duration of the timed pulse is justslightly smaller than the shortest transit time for the ultrasonic wavesthrough the fluid, in view of the variety of uids, and changes invelocity of ow intended to be measured.

Figure 1 shows a connection 25 from the mode suppressor to the inputcircuit of the R.F. amplifier, the connection being such to preventamplification of any received signals for the duration of the pulse, aswill appear more fully hereafter. The mode suppressor described isequally applicable to the form of oscillating loop shown in Figure l, aswell as to other broad band oscillating loops, such as the loop in whichthe modulator normally is olf A mode suppressor designed to provide aparalyzing pulse longer'than one-half of the transit time of theultrasonic vibrations between the transmitter 12 and the receiver 13will suppress all modes higher than the fundamental. Referring now toFigure 4, it is seen that the carrieramplitude must remain unbroken forthe period Tm (the time of the paralyzing pulse furnished by the modesuppressor). If a disturbance such as a noise pulse should take place,prior to the termination of Tm (such as dotted lines B in Fig. 4), thecarrier amplitude will not be interrupted and the form of oscillationwill continue in the normal fundamental manner as shown by the solidlines of the diagram. If the disturbance should take place after thetermination of Txn shown as solid lines B in Figure 5, the transmittedcarrier will contain the break since there is no longer any mechanism toprevent it. However,'at a 'time T from zero when the received image ofthelrising edge/of 0 ordinarily Would'cut oifthe Inodulator (dottedlineT), the modepsuppressor previously activated by the rising edge ofbreak B prevents this cutting off, and normal operation must proceedusing On as the effective new zero starting time for normal operation.It is, therefore, seen that the mode suppressor in this case enables theoscillator to reject a previous start which is improper and begin properoperation from the rising edge of a break. The suix .N in Figure 5 showsthe new time relationship upon proper operation of the loop. It can alsobe seen that there cannot be a perpetuated false break. Such a falsebreak cannot even be perpetuated if it is an ininitesimally smalldisturbance occurring directly after the conclusion of Tm, if time Tfalls within the next succeeding period Tm.

Therefore, if 2Tm=T, or Tm=1/2 T, all modes except the fundamental willbe suppressed. The lowest undesirable, symmetrical higher mode ofoscillation (for a circuit as illustrated in Fig. l) which may beperpetuated in a symmetrical manner, is the third harmonic mode. Thewave shape, but for the action of the mode suppressor, is shown indotted lines in Figure., This mode is suppressed by a slightly longerpulse than only one third of the frequency of the fundamental mode, asclearly appears from Figure 6.

Since the period of the paralyzing pulse is best adjusted to be slightlylarger than one half of duration of the pulse, it is to be noted thatuids having velocities of ultrasonic propagation of a range of almost 2to 1 may be measured without changing the time constant of the modesuppressor.

Reference will now be had to Figure 7 and 7a, where the circuit of themode suppressor and its relation to the iiow meter is indicatedindetail. Power supplies are omitted since they are conventional.Representative values of circuit elements are indicated on the drawing,where they are important for a complete understanding of the invention.Values of conventional circuit elements, such as bypass condensers,filter and load resistors, etc. are omitted for clarity.

The wave generator 14 (see also Fig. 1), which may be of any well knownconstruction, supplies excitation at ultrasonic frequency, for example,l0 mc. to modulator 15, which includes a tube connected as cathodemodulator. Tube 50, which may be a type 6AH6 pentode, has its controlgrid 51 biased for normal amplification, unless a signal is received byreceiving transducer 13, as

will appear more fully hereafter. The output of modulator tube 50,appearing at lead 52, drives another tube 53 (which may be a halfsection of a tube type 12AT7) connected as a cathode'follower. Tube 53acts as low irnpedence driver for the l0 mc. transmitting transducer 12.

The transmitting transducer 12 will then transmit ultrasonic vibrationsthrough the uid medium in pipe 11, to

the receiving transducer 13. The time elapsed between the start oftransmission of the first ultrasonic wave through the uid medium and itsreception by the receiving transducer will be a function of the velocityof propagation in the still fluid as well as of the flow velocity ofthek uid in pipe 11, and this elapsed time is used to measure thevelocity of the ow.

The receiving transducer 13, which is tuned to 10 mc., is connected bymeans of a coaxial 'R.F. cable to a resistance loaded tuning inductanceand then to the inputy of tube 54, which again maybe half of tube type12AT7.

Tube 54 is connected as a cathode follower. The output g of this tubeappearing at lead 55 feeds the input of R.F. amplifier 16, where shownas including tube 56. The plate circuit of tube 56 is a resonantinductance-resistance network 57, and drives a phase inverter circuit,well known in thel art. This circuit includes a tube unit 58 (whichagain may be a half section of a type 12AT7 tube) and is connected bymeans of leads S9 and 60 to a double diode tube 61, connected as afull-wave detector. Tube 61 may be, for example, a typeALS. The D.C.output voltage developed by tube 61 'is filtered in a filter circuit *f62 and applied by means of lead 63 to a tube unit `64, which is hereshown as the second section of 12AT7 tube used for tube unit 58. Thetube' unit 64 is shown connected as a cathode follower. This cathodefollower will supply a negative bias by Vmeans of lead 65 to grid 51 oftube 50 when a signal is impressed thereon, thereby cutting ofi tube 50and in turn cutting off the supply of excitation from the carriergenerator 14 to the transmitting transducer 12.

It is, therefore, seen that whenever a signal appears atV the receivingtransducer, the amplified and rectified signal places a negative voltagevon the grid` 51. Tube 50, therev fore, serves as a modulator.

The description of the circuit` of the flow meter has been given for aclear understanding of the operation of the mode suppressor now to bedescribed.

The mode suppressor 20 is connected by means of lead 24 to thetransmitting transducer (see also Fig. l). Lead 24 is connected to adiode detector 70, which may be of a type 1N34A. This diode detector70rectifies the signal appearing across the transmitting transducer. Ableeder resistor 72 connects the diode detector 70 to ground. Therectified signal will appear as a square wave impulse, whose frequencyis determined by the transit time of the ultrasonic vibrations betweentransmitting transducer 12 and receiving transducer 13. This impulse ofsquare wave shape is applied over lead 73, here shown as a shieldedcable, to the differentiating network 22, consisting of a condenser 74and a resistor 75. Lead 76 connects the output signal from thedifferentiating network to a monostable multivibrator 78. The rise involtage upon the appearing of a signal on the transmitting transducer,as rectified by detector 21, will trigger the multivibrator 78. Thetiming of the multivibrator 78 is determined by the fixed constants inthe circuit. The circuit of the multivibrator itself is conventional andis not further described in detail. The pulse forming the output of themultivibrator is taken by means of lead 25 and applied across a portionof grid return resistor 79 of tube 56, which is the R.F. amplifier. Thisoutput pulse will introduce a large negative voltage into the gridcircuit of tube 56 and disable or paralyze the operation thereof, sothat no amplification of any receiving signal can take place which, inturn, will block the application of any negative signal into themodulator tube 50, so that modulator tube 50 will permit excitation fromcarrier 14 to be applied to the transmitting transducer 12 for so longas the multivibrator 78 supplies a negative signal over lead 25. v

in addition to applying the negative block to tube 56, the output fromthe multivbrator is also taken over lead 80 to a tube unit 82, whichmaybe the second half of the l2AT7 tube containing unit 54. Tube 82 isconnected as a diode and operates as a D.C. restorer, connected in shuntwith resistor 79, so that only negative blocks from the mode suppressorwill appear across the resistor '79. The negative block applied to tube56 must be large enough to bias tube 56 to cut-off.

The timing of the multivibrator may be varied by varying the resistorcondenser network of the multivibrator circuit as is shown by the arrowsin Figure 7a and as is well known in the art.

A frequency lrneter 18 which measures the frequency of the rectifiedpulses of period 2T (or T, if the alternative circuit in which themodulator is normally off is used) may ne connected to lead 85. Lead 85is conveniently connected to a point in the circuit where a rectifiedsignal already appears, for example to detector 7@ as shown;alternatively the frequency meter may also be connected to the output ofdetector 17, lead 63, lead 65, or the frequency meter may be equippedwithits own detectory and connected anywhere in the circuit. If thefrequency of the loop is to be'compared with the frequency of a similarloop, but of different spacial orientation with respect to the rstloop,then lead 85 may be connected to such comparator circuit 19.

It is therefore seen that I provide apparatus for suppressingundesirable disturbances in broad band oscillatory circuits by obtainingan impulse over lead 24 from the oscillatory circuit upon starting ofoscillations. The thus obtained impulse is differentiated in network 22,and the differentiated impulse is utilized to control a timed circuit23. Circuit 23 furnishes a pulse of accurately timed duration which isapplied over lead 25 to control the oscillatory circuit in such a mannerthat reintroduction of the disturbance into the circuit will beprevented so that the disturbance willbe suppressed.

It is understood that various changes may be made in the circuits, andthe specific embodiment described in detail is illustrative only.

What is'claimed is:

l. In a ow meter of the ultrasonic type having a signal generator, atransmitting transducer connected thereto, a receiving transducer, saidtransducers being arranged in acoustic contact with a uid so thatacoustic waves are propagated therebetween in a direction having acomponent parallel to the direction of flow of the fluid, an amplifierconnected to said receiving transducer, and a modulator connected to,and controlled by received signals from said amplifier, said modulatorbeing connected between the signal generator and the transmittingtransducer for affecting the signals furnished the transmittingtransducer by said signal generator; a connection from the transmittingtransducer; a detector connected to said connection; a differentiatingnetwork connected to said detector; a timed pulse generating circuitconnected to said differentiating network and furnishing a pulse uponbeing triggered by an impulse from the differentiating network; andcircuit means applying the pulse from the pulse generating circuit tothe amplifier to paralyze operation thereof for a predetermined timelessthan the period required for the waves to traverse the uid andthereby prevent a spurious received signal from affecting the signalsfurnished by the signal gent erator to the transmitting transducer.

2. In a flow-meter having a carrier generator, a transmitting transducerconnected to said carrier generator, and a receiving transducer, a fluidconduit, said transducers being mounted opposite each other in said uidconduit so that waves are propagated between said transducers in adirection having a component parallel to the direction of fluid flow; agating circuit interposed between the carrier generator and thetransmitting transducer, said gating circuit normally permitting energyto pass from the carrier generator to the transmitting transducer; andconnection means from the receiving transducer to said gating circuit toaffect the condition of said gating circuit to block the transmission ofenergy v from the carrier generator to the transmitting transducer whena signal is received by the receiving transducer, spurious modesuppession means responsive to the output of said gating circuit forpreventing said connection means from affecting said gating circuit fora predetermined time less than the period required for said waves totraverse the fiuid, and frequency measuring means connected to saidgating circuit.

3. In a How-meter according to claim 2, wherein said spurious modesuppression means includes a detector connected to the transmittingtransducer, a timed pulse generating circuit connected to said detectorto furnish a pulse, and means to apply said pulse to the gating circuitto prevent the gating circuit from being affected by a signal receivedby the receiving transducer.

4. A flow-meter of the ultrasonic type comprising an ultrasonicfrequency wave generator means having an oscillating and anon-oscillating condition, a fluid conduit, transmitting and receivingtransducers mounted in said fluid conduit so that acoustic waves arepropagated through a ud in the conduit from the transmittingtransducer-to the receiving transducer in a direction having a componentparallel to the direction of `uid flow, means connecting thetransmitting transducer to the output of the wave generator means andmeans connecting the receiving transducer to the Wave generator means,circuit means connected to said generator and said transducers, saidcircuit means comprising means for producing a direct current blockingpotential in response to the oscillating condition of said generatormeans and means for causing the blocking potential to prevent the wavegenerator means from changing from one condition to the other during thetime that the blocking potential continues and frequency measuring meansconnected to said generator means.

S. A flowmeter of the ultrasonic type comprising a transmittingtransducer, a receiving transducer, an ultrasonic frequency wavegenerator means and circuit means connected to said Wave generator meansand transducers, means for mounting said transducers with respect to aiiuid so that acoustic waves emitted by the transmitting transducer arepropagated through the fluid to the receiving transducer in a directionhaving a component parallel to the flow of the fluid, said circuit meansincluding means for controlling the Wave generator means in response tosignals received from said receiving transducer, means `f0.1', producinga direct current blocking potential having a duration greater thanone-half the `travel time ofthe acoustic Waves through the fluid inresponse to oscillations of said generator means and means responsive tosaid blocking potential for preventing the received signals fromcontrolling the wave generator means for the 'duration of said blockingpotential'and frequency measuring means connected to said 'circuitmeans.

